Classic Motorcycle Build

Electrical Wiring For Motorcycles

Electrical problems are one of the biggest challenges when building a classic or custom bike.
Unlike mechanical issues, what's not working may not be visible to the eye.

Motorcycle Electrical Systems

There are three major parts of a motorcycle electrical system - primary, secondary, and charging system.
What hooks them all together is wiring, switches and relays.
So let's break these down into smaller categories.

Low-Tension Side

In a points-operated ignition system, the primary, or low-tension side of the ignition system starts with the battery, goes to the ignition switch, then to the primary windings of the coil.
A key-operated ignition switch connects or disconnects the power.

Battery

Ignition switch

Coil (primary winding)

High-Tension Side

The voltage from the primary windings of the coil go into the secondary windings of the coil, where they come out at over 20,000+ volts.
An ignition coil is basically a step-up transformer.
The secondary, or high tension side of the ignition system goes to the contact breaker (points), which gives the necessary volts to the spark plugs.

Coil (secondary winding)

Points and Condenser

Spark Plug Wires

Spark Plugs

Best Primary Wire for Motorcycles

Most classic motorcycles use 16-gauge or 18-gauge copper wire insulated with plastic.
For starter and generator wires, a heavier 14g or 12g is needed.
Heavier 12-gauge wire is required when you need something that can handle a bit more load.
You can use standard automotive-type wire found in auto part stores, but there is a difference between SAE-rated wire and AWG (American Wire Gauge) sizing.

AWG Rated Wire

Voltage drop is the wire's ability to carry current with a minimum amount of electrical resistance.
AWG sizing always has more copper for a given gauge size, and more copper is better.
This variation is about 10-20%.

Use quality connectors or solder joints to connect wires to terminals.
If you choose crimp connectors, take off the colored plastic caps and throw them away.
Seal all connections and terminals with shrink tubing.

Make sure that the battery and motor are well grounded to the frame.

On several of my motorcycle projects, I started from scratch with nothing but a simple wiring diagram drawn on a piece of paper.
I usually start by mounting the battery, then adding the other electrical components one at a time; headlight, taillight, ignition switch, coil, etc.

Making A Wire Harness

Wires and their connections develop oxidization over time, which lead to poor connectivity and eventual failure.
Replacing a single wire or connector may fix your electrical problem, but if this happens several times, consider rewiring the entire bike.

To make a wiring harness, or completely rewire a bike, you will need:

Wire

Wire cutters/strippers

Soldering gun and soldering wire

Multi-meter and/or Test-Light

Drawing a wiring diagram out in a notebook (no matter how simple) helps visualize what needs to go where.
It should show where each wire is starting, where it will end up, and what color it will be.
Abbreviations for wire colors are usually "BK" for black and "BL" for blue, "R" for red, etc.
While rewiring your motorcycle, take pictures of everything for future reference.

I prefer to route any length of wire in either cloth or plastic wire looms.
Some builders like wrapping wires in electrical tape, or only use heat shrink tubing on the ends.
All these methods will work, what's important is that the wires are secure and are away from any sharp edges.
They should also be kept away from heat (cylinders and exhaust).

6-Volt to 12-Volt Conversion

Harley-Davidson used a 6-volt electrical system on motorcycles until 1964 and on trikes until 1963.
If your 6-volt battery and electrical system are in good shape and the motor has not been extensively modified, you should not have any problems with the original 6-volt system.

A 6 volt wiring system and switches are more than enough to handle 12 volts. This is because it's heavier because 6v has to carry more amperage than 12v. Less volts means more amperage. Changing to 12v cuts the amperage load approximately in half.
So there's nothing wrong with using 6-volt wiring in a 12-volt system.

6v to 12v Gauges

If your gauges are mechanical, there's no problem. If they are electrical, they will need a voltage reducer to make them work.
If you have warning lights on your dash, the bulbs must be changed.

Headlight, tail-light, parking lights, dash lights, etc., all 6 volt bulbs must be replaced with 12 volt bulbs.
You can use your headlight switch, brake-light switch, and high-beam switch. They will work in either a 6v or 12v system.

Generator Charging Problems

Your classic bike's generator really doesn't charge the battery until engine rpm is considerably higher than idle.
If your bike is electric-start, trips that are less than 15-20 miles are typically not enough to recharge the battery's losses from starting.

Simple Generator Test

The first sign of a generator not charging properly is a dead battery.
The simplest way to test a generator is, with everything hooked up properly, start the engine.
Then see if the headlight gets brighter when you rev the motor from idle.

Another way to test generator output is to compare the difference between voltage at the generator and voltage at the battery.
There should be higher voltage at the generator.

Motorcycle Battery Care and Maintenance

A motorcycle battery has a harder life than a car battery.
Not only are the plates inside them more fragile, but they are subject to much greater vibration.

Battery cables are the most critical wires on your motorcycle.

Getting tired of buying a new motorcycle battery every couple years?
Seems we neglect them until there's a problem, unnecessary damage is done, and we're spending money for a new one.

Re-wire Handlebars

Handlebars are one of the first things riders change. If you're thinking of installing taller handlebars, you'll need to extend the switch wiring harness.
Many wire extension kits are available that have all the right colors and terminals for your year and model.
Or you can be creative and make your own.